Bragg gratings were first photoetched in optical fiber cores or in planar waveguides in 1978 by K. O. Hill who applied ultraviolet irradiation (190 nm to 250 nm) transversely to optical fibers. That technique is described in document [1].
These components are presently under intense development due in particular to the multiplicity of applications in which they are used.
They are thus to be found in the field of optical telecommunications, e.g. for wavelength division multiplexing, for compensating chromatic dispersion in optical fibers, for stabilizing and flattening the gain of optical amplifiers, for stabilizing the frequency of semiconductor lasers, and more generally in optical fiber lasers and in various filters.
They are also to be found in the field of sensors, e.g. strain or temperature sensors.
A major drawback of Bragg gratings is that they present characteristics that are sensitive to temperature variations.
Thus, the Bragg wavelength .lambda..sub.B of a uniform grating is given by the following relationship: .lambda..sub.B =2n.sub.eff .times..LAMBDA. where n.sub.eff and .LAMBDA. designate respectively an effective refractive index of the guided mode and a pitch of the grating.
Under the effect of a temperature fluctuation .delta.T, the wavelength .lambda..sub.B of a free grating is subject to a variation .delta..lambda..sub.B given by the relationship: EQU .delta..lambda.B/.delta.T=(.alpha.+.zeta.)..lambda..sub.B
where .alpha. and .zeta. represent the coefficient of thermal expansion and the thermo-optical coefficient of the core glass, and generally have respective values of 0.5.times.10.sup.-6 K.sup.-1 and 7.times.10.sup.-6 K.sup.-1.
Thus, for a free etched grating having a wavelength at 20.degree. C. and in a non-prestressed state of about 1550 nm, the Bragg wavelength .lambda..sub.B changes by about 1.2 nm for a temperature change of 100.degree. C.
In numerous applications, variations in the characteristics of gratings due to temperature fluctuations are to be avoided, so it is necessary to provide devices that enable that drawback to be remedied.
For example, in the field of optical telecommunications, it is generally necessary to obtain gratings having characteristics that are highly stable, and that remain so over a temperature range that can extend from 20.degree. C. to +80.degree. C.
Various devices have been proposed seeking to stabilize the Bragg wavelength or resonant wavelength of a Bragg grating relative to temperature.
Active systems have been proposed, for example devices including Peltier type elements, in which the Bragg wavelength is measured continuously and corrected by applying mechanical stresses.
Such systems are expensive and bulky.
In general, preference is given to so-called passive devices.
Two types of passive device are presently known that enable the temperature sensitivity of the Bragg wavelength of gratings to be reduced.
A first type of device has a support material with a negative coefficient of thermal expansion and the Bragg grating is stuck thereto with a selected amount of pretension.
When temperature rises, the support material with the negative coefficient of expansion contracts, thereby reducing the initial pretension applied to the grating while it was being fixed thereto. The support material therefore tends to stabilize the Bragg wavelength around its initial value.
Thus, proposals are made in document [2] to use an oriented copolymer as the material having a negative coefficient of expansion. In document [3], proposals have been made to use certain glass-ceramics as the support material, however fabrication thereof is still at the laboratory stage.
Devices of the first type suffer from two major drawbacks. Thus, the coefficient of thermal expansion must be accurately matched and must be constant from one sample of the material to another, which is difficult to achieve in practice. Secondly, it turns out to be difficult to machine such materials without spoiling the properties thereof, and in particular without altering their coefficients of thermal expansion.
A second type of device for temperature stabilizing a Bragg grating operates on the principle of differential expansion. In such devices, two elements are used whose component materials have coefficients of thermal expansion that are very different, so that when temperature rises, pretension initially given to the grating is relaxed.
Documents [4], [5], [6], and [7] thus propose associating aluminum as the material having the greater coefficient of expansion with Invar, silica, stainless steel, or iron as the material having the smaller coefficient of expansion.
Those devices are particularly lengthy and difficult to make. Fabrication requires a large number of technological steps. Furthermore, those devices can have a large number of points of adhesive, in particular between metal and glass, and the long-term behavior of such bonds turns out to be unreliable.
Making such devices also requires both the pretension applied to the grating and the spacing between two end fixing studs for a fiber having a Bragg grating to be accurately controlled. Such control is extremely difficult to perform with the desired degree of precision.